A flat monolithic accelerometer conventionally comprises a body having a base and two measuring cells for the purpose of differential measurement. The flat structure allows simple and economic manufacturing, notably by chemical etching methods. A measuring cell typically comprises a seismic mass connected one the one hand to the base and on the other hand to a force sensor which is itself also connected to the base. When the accelerometer is subjected to an acceleration along the sensitive axis which is the axis of the acceleration to be measured, the seismic mass is subjected to an inertial force which is amplified and transmitted to the force sensor by means making it possible to amplify the transmitted force or displacement.
In a known way, the amplification is obtained by means of an arm called a lever arm which extends the seismic mass. The displacement of the seismic mass is transmitted to the force sensor by means of this lever arm. More precisely, the arm is connected to the base by an articulation making it possible for the mass to rotate about an axis perpendicular to the sensitive axis of the accelerometer and is connected to the force sensor by a hinge. When the accelerometer is subjected to an acceleration along the sensitive axis, the seismic mass is subjected to a force which makes it rotate about the articulation as therefore does the part of the lever arm connected to the force sensor.
The force sensor is a vibrating beam or beams sensor. The vibrating beam is connected to electrodes making it possible to make it vibrate at its resonant frequency and to a circuit for measuring the variation of its resonant frequency.
The measuring cells are mounted in such a way that when the accelerometer is subjected to an acceleration along the sensitive axis, one of the beams undergoes a traction force, the other beam undergoing a compression force of the same value, this traction or this compression causing the resonant frequency of the beam measured by the measuring circuit to vary. A differential measurement is thus obtained notably making it possible to avoid certain non-linear effects.
The variation of the resonant frequency is directly related to the displacement of the force sensor induced by the rotation of the part of the lever arm connected to the force sensor. The end of the beam also undergoes a certain rotation which often proves to be annoying, notably in the case of a tuning fork (that is to say of two beams forming a tuning fork) where the force transmitted to the two beams is not exactly identical.
Moreover, the machining quality of the hinges and articulations is of prime importance and constitutes one of the industrial limitations of this accelerometer.
In addition to the displacement being proportional to the length of the lever arm, the overall dimensions become greater when it is desired to obtain a large amplification ratio.
The French patent application FR 2 848 298 (THALES) proposes an accelerometer, such as shown in a simplified diagrammatic manner in the appended FIG. 1, whose amplification means MA1, MA2 do not comprise a lever arm used in rotation and generally comprise a respective resonator R1, R2 which can be a vibrating beam. This document describes an accelerometer ACC micromachined in a flat plate comprising a base and at least one measuring cell comprising a mobile seismic assembly ESMC comprising a mobile seismic mass connected to the base and able to move in translation along the sensitive axis Oy of the accelerometer under the effect of an acceleration γ along this axis Oy. A resonator cell comprises a resonator able to vibrate and to undergo a traction or a compression, depending on the direction of the acceleration γ and placed symmetrically with respect to the axis of symmetry S of the structure, this axis S being parallel with the axis Oy and passing through the center of gravity of the seismic mass. A measuring cell furthermore comprises means MA1, MA2 of amplification of the acceleration force generating the translation comprising at least one foot-piece PA1, PA2 for anchoring to the base, two rigid end-pieces of the resonator cell and two pairs of micromachined arms, the pairs being symmetrical with respect to the axis S, each pair comprising a first arm connecting a first point of attachment to an end-piece and a second point of attachment to the seismic mass, and a second arm connecting a third point of attachment to the same end-piece and a fourth point of attachment to the anchoring foot-piece, the angle α between the axis Ox perpendicular to the axis Oy. The line joining the first and second attachment points is symmetrical, with respect to the axis connecting the end-pieces through their center, with the angle between the axis Ox and the line joining the third and fourth attachment points and sufficiently small for the force applied in traction or in compression to the resonator to be greater than the acceleration force applied to the seismic mass.
Because of the symmetry of this structure, the displacements of the seismic mass, of the attachment end-pieces and of the resonator are perfectly axial. Moreover, the performance of this structure, that is to say the amplification ratio obtained, is simply determined by the angle α; the geometry of the seismic mass, whose center of gravity is situated on the axis of symmetry S, has no effect on the performance of the accelerometer.
The amplification means described can be of the so-called “jack” or “butterfly” shape. FIG. 1 shows an example embodiment with amplification means having the “jack” shape.
Such a system comprising a single mobile seismic mass is relatively sensitive to thermal expansions of the materials outside of the measuring cells. Thus, the quality of the measurements of the accelerometer can be greatly degraded as such thermal deformations increase. Moreover, when the temperature of the structure is not homogeneous, parasitic stresses degrading the measurement can appear.
Systems are also known comprising two seismic masses MS1 and MS2, each connected to the base of the accelerometer, as shown diagrammatically in FIG. 2. Such two-mass systems are very sensitive to manufacturing spreads, notably with regard to the differential response of the two measuring cells in the presence of vibrations, the vibration mode of one mass, called the spring-mass mode, and the excess tension of the first detector can be different from those of the other mass, which has a considerably harmful effect on the differential gain in the presence of vibrations close to these modes.
Such systems are sensitive to manufacturing tolerances and to the thermal expansions undergone by the accelerometer and notably the attachment points.